The present invention relates to a process and apparatus using wood wool or any other ribbon lignocellulosic type material as furnish for the production of low density composite panels with improved moisture or/and water resistance. More specifically it relates to the fabrication of commercial composite panels of very low density and to the panels so produced. Furthermore, thick panel products i.e. panels with a target thickness greater than xc2xe inch (19 mm) may benefit greatly from this invention.
Wood-based panel composites including oriented strand board (OSB) and medium density fibreboard (MDF) are widely employed as substitutes for solid wood in many applications. To produce wood-based composite panels, resin and wax are applied onto furnish prior to mat formation followed by hot pressing for resin curing panel consolidation. The purpose of hot pressing is first to density the panel then to provide sufficient energy to the resin to polymerise it and hence develop an effective bond for panel consolidation. The wax performs at least two important roles in the composite wood panel. Besides improving the flowability of the resin during hot pressing, the wax improves the dimensional stability of the resultant panel.
There is no doubt that the cost and speed of production of composite panel products is time and furnish dependant, and, there has been observed a significant increased interest in this subject through the advances in resin and manufacturing technologies. However wood is low in heat conductivity, limiting the heat transfer efficiency from the platen to the core of panel. Hence longer hot pressing periods are required particularly for thick panels. For example, to produce a good quality xc2xe inch thick OSB panel will usually require a press cycle of more than 5 minutes.
It has been recognised that furnish compaction ratio, which is directly related to furnish bulk density, plays an important role in the mechanical properties of the finished panel. The mechanical properties of panels made from high compact ratio density are generally better than panels made from low compact ratio furnish.
It also been recognised in the art that steam is very good in heat conductivity and that the pressing time for particle board or like products can be drastically reduced by passing pressurised steam through the pressed panel, or simply by increasing the furnish moisture content. Special resins, however, are required here to prevent resin from being hydrolysed or washed-out during hot pressing.
The invention provides a method for manufacturing a low density composite wood panel, comprising:
(a) providing a feed stock comprising wood strands having the following characteristics:
an average slenderness ratio of from 200 to 800;
an average aspect ratio of from 50 to 500; and
a bulk density of from 0.08 to 0.02 grams/cm3.
(b) blending said feed stock with a resin binder to provide a resinated furnish;
(c) forming said resinated furnish into a mat; and
(d) hot pressing said mat to form a finished panel.
The feed stock wood strands employed can be the waste product from the production of wood-wool for special applications in the packaging industry.
Acceptable dimensions of wood-wool average about 0.25 mm in thickness, 2 mm in width and up to 400 mm in length. This corresponds to a slenderness ratio of up to 1600, an aspect ratio of up to 200 and a bulk density of about 0.01 g/cm2.
The invention also provides a low density composite wood panel comprising a furnish mat formed from wood strands having the following composition:
an average slenderness ratio of from 200 to 800;
an average aspect ratio of from 50 to 500; and
a bulk density of from 0.08 to 0.02 grams/cm3;
said mat including a resin content in the range of 3-5% by weight and being hot pressed to form a finished panel. Existing wood-wool making machines can be adjusted to provide efficient control of the thickness and the width of the wood-wool strands, but, because of the strand length breakdown during processing, are unable to control the length of the strands. Thus, a screening process to separate a product wherein the strands are of a desired range of length is required. This is done by use of a vibrated screening technique and short strands that pass through the screen holes are discarded from packaging applications. The discarded waste is reclaimed, to provide strands usable for wood-wool panels and varying in length from about 15 to about 400 mm. This wide range in length of the furnish strands utilized accounts for the broad ranges expressed above in respect of the slenderness ratio, aspect ratio and bulk density.
Preferably a hot wax is added to the furnish by spraying prior to addition of the resin, the wax content being in the range 1 to 2% by weight of the furnish. As described, the purpose of wax addition in the composite wood panel is not only to improve the flowability of the resin, but, also to improve the dimensional stability of the resultant panel.
The resin content of the furnish is a function of resin type an panel type to be made. For a typically OSB it is in the range 2 to 3% by weight, and preferably for wood-wool panel it is approximately 4%. Generally speaking, the mechanical properties of the OSB panel decrease with density, and to improve product quality a higher resin content in the range 3 to 5% is preferred.
The type of resin used is a function of the end application of the panel. For exterior grade panels MDI or high molecular phenol-formaldehyde resin is preferred. Urea-formaldehyde (UF) and melamine urea-formaldehyde (MUF) are suitable for interior grade panels.
The selection of pressing technique is independent of resin type to be employed. Both pressing methods are suitable for UF, MUF, MDI and novolac PF. Therefore, because of the high humidity condition MDI or high molecular weight PF are recommended for conventional hot pressing. This may prevent resin from being hydrolysed or washed-out during hot pressing.